Vacuum conveyors are usually used for transporting small-particle or powdery material, such as powders, granules, dusts, tablets, or small parts in a suction gas flow, which, as a rule, consists of air or inert gas. The material is suctioned off from a supply site via, for example, a manual suction tube or a feed funnel, is transported through a hose or tube conduit, and arrives at the separation chamber of the vacuum conveyor via a suction opening. There the suction gas is separated from the transported material, which is collected in the separation container, and if need be, a filter element is downstream from the gas side. The suction gas leaves the separation chamber in the direction of the vacuum pump, which is usually fixedly installed directly on the upper side of the separation chamber.
In a standard vacuum conveyor, a discharge valve, through which the material can leave the separation chamber, to fall directly into a packaging container, is as a rule located on the underside of the separation container. In addition, a gas valve can be situated between the separation container and the vacuum pump.
During the suctioning process, the so-called suction cycle, the discharge valve is closed and the gas valve between the separation container and vacuum is opened. The separation container is thus filled for a certain prespecified time or until a specific degree of filling with material has been reached. Subsequently, in the so-called discharge cycle, the gas valve between the separation container and vacuum is closed or the vacuum pump is shut off and the discharge valve is opened. The transported material then falls, in the simplest case, through the discharge opening from the separation container. If necessary, however, the material can also be blown out from the separation container by excess pressure, or bridge-building materials can be fluidized. By counter-blowing from the pure gas site, the filter can also be cleared of adhering filter cake. A cycle control regulates the duration of the individual suction and discharge cycles.
Some vacuum conveyors also permit, in addition to simple transporting, the possibility of metering conveyed material. Above all, such vacuum conveyors with a metering function differ from vacuum conveyors without a metering function in that after switching off the vacuum pump or interrupting the suction, the discharge valve remains closed for sufficient time that the conveying process in the separation container comes to a complete standstill; thus, all of the material which was still in suspension has settled in the separation container. Afterwards, the conveyed quantity of material is weighed via a weighing device. This is done, for example, such that the vacuum conveyor rests on a weighing device, such as a frame construction with a system of, for example, three working-load weighing cells. This measures the total weight of the vacuum conveyor plus the conveyed material contained therein.
Subsequently, the discharge valve is opened. Afterwards and/or beforehand, the tare of the vacuum conveyor is weighed in order to determine the quantity of conveyed material removed. This repeated weighing is undertaken because experience shows that a one-time tare at the beginning of the conveying process is not sufficient. This is because the mass of conveyed material adhering to the filter of the vacuum conveyor and to the walls of the separation container changes from conveying cycle to conveying cycle, and thus falsifies the measurements.
In order to attain a larger metering quantity of conveyed material than is possible in one suction cycle, the material is conveyed in several suction cycles. A controller determines beforehand the number of required suction cycles and the theoretical filling quantity per suction cycle. Usually, in the first cycles, larger quantities of material are conveyed with rapid filling cycles; in the last cycles, if desired, operation is in the flying conveyance mode, guaranteeing a very continuous conveyance so that one can better approximate the nominal conveyed quantity. This process is very time-consuming.
Therefore, the procedure then became one of weighing the vacuum conveyor continuously during conveyance. Shortly before attaining the theoretical filling quantity for a specific suction cycle, the vacuum pump was switched off or the suction process was interrupted. This procedure is faster, but weighing accuracy suffers under the changing transverse forces that act on the separating container during the conveyance operation. In actual practice, it has become evident that the metering accuracy of these vacuum conveyance devices is often insufficient. Depending on the conveyed material, deviations result of 20 to 500 g from the actual conveyed quantity to be attained (target quantity).
FIG. 1 shows a view of a prior vacuum conveyor 100 with a metering function. As indicated in FIG. 1, the vacuum conveyor sits in a frame construction 120 on a 3-point weighing apparatus 150 and carries a vacuum pump 170 on its separation container 110. Since the vacuum conveyor is continuously weighed during the conveyance process, the weighing must suppress or minimize disturbing forces and influences as much as possible.
To this end, among other things, the entry side of the separation container was provided with an isolation system 140. As can be seen in FIG. 1, an isolator is connected to the entry opening of the separation container; its other end is firmly connected with the frame construction, where the (nondepicted) hose or tube system of the working side is connected. This isolation system is intended to guarantee that the forces generated by the transported material do not have a disturbing effect on the weighing.
It has become evident, however, that these measures are not sufficient to limit influences disturbing the weighing to an acceptable level. These disturbing influences derive essentially from the recurring differences in vacuum level within the transport conduit, whose disturbing transverse force influence cannot be compensated for with the previously known methods, in spite of a force isolation. The force isolation serves rather to avoid impacts and mechanical tensions that are generated by the conveyed product and the inherent weight of the transport conduit. Since determination of the timepoint for shutting off the vacuum pump or interrupting the suction process is done on the basis of weighing the vacuum conveyor, this point is arrived at only insufficiently among previous vacuum conveyors. The stipulated target value for the conveyed quantity is not attained precisely enough, as the deviations further discussed above already make clear.